Thermostat



Feb. 6, 1934. L. SATCHWELL 1,945,706

THERMOS TAT Filed March 4, 1951 2 Sheets-Sheet l #G M MJ M dbtouwq s Feb. 6, 19 34. SATCHWELL THERMO STAT Filed. March 4, 1931 2 Sheets-Sheet 2 ATTORJKEYS Patented Feb. 6, 1934 UNITED STATES PATENT OFFICE Application March 4, 1931, Serial No. 520,121, and in Great Britain November 12, 1930 6 Claims. (Cl. 200-137) This invention relates to thermostatic switches of the type designed to perform the switching operation on establishment of flame in the furnace and depends upon the maintenance of combustion to keep it in operative condition.

In accordance with the present invention which makes use of elements having different mass/ area relations to produce variation in the rapidity of expansion of two members in known manner, the two members have slightly differing coefiicients of expansion and are so arranged in relation to the switch that on sudden rise of temperature one of the elements'moves the other to a position to allow the switch to open but on continued rise of temperature the other element is brought into such relation to the switch that on sudden fall of temperature they are ready again to operate the switch without delay. The slight difference in expansion coeflicients of the two members permits the switch to act as an ordinary thermostat with slow changes of temperature. Thusthe mechanism may be positively actuated in one direction at the low temperature and positively actuated in the other at the high temperature regardless of the rate of temperature change.

The theory of this device is illustrated in Fig. 1 of the accompanying drawings. This figure consists of a pair of coordinates upon which the expansion of each of the two heat sensitive elements is plotted against time. In this figure the relative expansion of the two elements is represented by the vertical distance while the time elapsed is represented by the horizontal distance. The diagram illustrates the two operations common in any type of heater. First, going from left to right, the heater starts cold, the burner is turned on and the temperature rises. Second, when the burner is turned off, the temperature falls. The curve 1 indicates the relative expansion of the more sensitive element and this expansion closely follows the rise in temperature of the heater. The curve 2 shows the expansion of the more slowly acting element and it will immediately be seen that this curve is more gradual than the curve 1, thus indicating that the expansion of the slowly acting element takes pTace slower and lags behind that of the more quickly acting element. It will be noted that at the start the slowly acting element is slightly higher than the quickly acting element. This indicates that the slowly-acting element is slightly longer when the furnace is cold. As the temperature rises, however, during the progress of time, the quickly acting element responds more rapidly and soon becomes expanded beyond the slowly acting element. As soon as this difierence reaches a sufficient amount it is utilized to operate the control mechanism to prevent the burner from being turned ofi. This point is indicated by the small letter a. As the heater reaches maximum temperature and remains constant for a considerable period of time both elements reach their final state of expansion and I find at points 3 and 4 that the expansion of the elements has practically ceased. At this point it will be noted that the curve 1 is slightly above the curve 2, thus indicating that conditions are reversed to what they were at the beginning and the quickly acting element is slightly longer relatively than the slowly acting element.

After a break in the diagram which denotes the lapse of any length of time the reaction of the device is shown upon the extinguishing of the flame. When the flame is extinguished the temperature of the furnace begins to drop rapidly at first and then slower as a return is made to room temperature. A very similar drop is taken in the expansion curve 1 of the quickly acting element. The expansion curve 2 of the slowly acting element lags as before and instead of a sharp drop a long relatively gradual drop is made. Thus the curve 1 crosses the curve 2 and the difference betwen the two elements is reversed thus operating the control mechanism at a point such as b to turn off the burner. From this diagram it can easily be seen how short is the lapse of time before the desired reactions take place.

A third point e is marked on the curve 1 and this point 0 represents the place on the cooling curve which corresponds in temperature to the point a on the heating curve. Should an ordinary thermostat be used in place of the applicants device the burner would not be turned off until the heater had cooled to point 0 and it is apparent from the diagram how much time this would take in proportion to that taken by the applicants device.

In the diagram referred to a difierence in the expansion coeflicients of the two elements is shown. However, it is obvious that the curves 1 and 2 might coincide exactly at the upper and lower temperatures, that is that the expansion coeflicients might be exactly the same.

In accordance with one embodiment the sensitive member consists of two parts anchored to one another, an outer shell which may be a split tube or two or three vanes of metal and a central tube or rod. The co-efiicient of linear expansion of the outer shell or the like is preferably, though not necessarily slightly greater than that of the central tube or rod.

Figs. 2 and 3 illustrate this thermostat in diagrammatic form. Fig. 2 shows the thermostat when the heater is cold and Fig. 3 shows the thermostat when the heater is hot. In these figures a thin or otherwise quickly responsive element 10 is fastened at one end to a stationary support 11 and at the other end tothe thicker slowly responsive element 12. The two elements lie in such positions that upon expansion the expansion of one tends to neutralize that of the other. At the opposite-end of the element 12 from that at which it is fastened to the element 10, the element 12 is operatively connected through an adjusting screw 13 to a switch 14 or other controlling mechanism. As shown, the switch 14 comprises a pivoted lever 15 carrying a contact 16 adapted to make contact with a stationary contact 1:'. This contact may be made and broken in accordance with my co-pending application Serial Number 234,651, filed November 21, 1927.

Instead of an electrical switch the expansible elements may operate any other control mechanism such as an air valve or a mechanical conrol.

Although the two elements have been shown diagrammatically in one of the preferred forms in Figs. 2 and 3 they may, however, be disposed side by side, anchored at the end remote from the switch and connected at the switch end with a rocking member for operating the switch.

A thermostat for the safety control of an oil, gas or powdered fuel furnace embodying a thermally sensitive member in accordance with the invention and consisting of an outer shell and an inner tube is illustrated by way of example in the accompanying drawings in which Fig. 4 is a side elevation, part of the outer'sensitive member being broken away, the better to illustrate its construction. Fig. 5 is a section on the line XY of Fig. 4.

Referring to the drawings, A indicates the cover of a switch (not shown) adapted to be actuated by the heat sensitive member which consists of two parts, viz an outer shell B in the form of a split tube which is formed with ribs C for strengthening purposes, and a central tube or rod D. The outer shell B is preferably of a metal having a slightly greater coefficient of linear expansion than the central tube D. The shell E is anchored to the tube D at its end remote from the switch and is secured at its inner end by welding to a sleeve E supported from the switch head on a plate F spaced therefrom. It is free to expand only in the direction away from the switch head while the central tube D extending freely through the sleeve E is connected to a screw threaded adjusting rod H (screwed into the tube D) adapted to operate the switch. The tube D is free to expand towards the switch head.

On the heat sensitive element being first heated by the flame the outer shell B rises in temperature more rapidly than the central tube D, the outer shell B expands in the direction away from the switch head, taking with it the central tube D and thus the rod H allowing the switch to 0perate. Continued application of heat results in both the outer shell B and its central tube D becoming hot to the same degree the tube D expanding towards the switch head from its point of attachment to the outer shell so that the rod H returns almost to its cold position in a position removed from its initial cold position by a difference due to the diiference'in the coeflicient of expansion of the outer shell and tube. This distance is just'sufiicient to allow the switch to remain in operated position. Should the source of heat be removed, i. e. should the flame in the furnace fail, the outer shell will drop in temperature more rapidly than the inner tube and will contract, moving the central rod inwards and pushing the switch into its composition.

This construction has proved exceedingly satisfactory in conjunction with thermostats which employ contacts under the influence of a magnet, because the magnetic cling can be adjusted so that a definite dimension difference must exist between the expansion elements to cause opera tion of the switch.

Fig. 6 is a partial sectional view showing in detail the adjusting screw H and the parts coopcrating therewith.

From this it will be seen that the adjusting screw H forms the operating connection between the rod D and the control mechanism by extending through the housing A and bearing at a point I against the control mechanism (not shown). The other end is threaded into the rod D. An adjusting arm K,, fixed to the screw H, adjusts the angular position thereof so as to screw the screw H into or out of the rod D. A lock nut L holds the screw, H in the position in which it is set.

It will be understood that the above illustration is cited merely by way of example and this invention is limited only as indicated by the scope of the following claims.

I claim:

1. In a thermostatic switch, an expansion member comprising two longitudinally expanding parts secured together at one end, each part having an expansion coeflicient, the expansion coefficients of said parts diflfering not more than by slight values, the mass area values of the parts of the expansion member being different and so related to the coeflicients of expansion of said parts that upon a rapid change to a difl'erent temperature a temporary difference of dimension occurs which, to a major extent, is eliminated upon that different temperature being maintained.

2. In a thermostatic safety switch, an expansion member comprising two longitudinally expanding parts secured together at one end, each part having an expansion coefficient, the expan sion coeiiicients of said parts differing not more than by slight values, the mass area values of the parts of the expansion member being different and so related to the coefficients oi. expansion of said parts that upon a rapid change to a different temperature a temporary difference of dimension occurs which, to a major extent, is eliminated upon that different temperature being maintained, the slight difference in the expansion co eflicients oi the parts of the expansion member serving to maintain the switch in operative position when both elements have reached the same temperature.

3. In a thermostatic safety switch, an expan sion member comprising two longitudinally expanding parts secured together at one end, each part having an expansion coefiicient, the expansion coefficients of said parts differing not more than by slight values, the mass area values of the parts of the expansion member being different and so related to the coeflicients of expansion of said parts that upon a rapid change to a different temperature a temporary difference of dimension occurs which, to a major extent, is eliminated upon that different temperature being maintained, a contact member and means for operating the contact member in one direction by the parts of the expansion member, the slight difference in the expansion coeflicients bringing the switch into a position ready to be operated in the reverse direction immediately following a sudden fall of temperature.

4. In a thermostatic switch, an expansion member comprising two longitudinally expanding parts secured together at one end, each part having an expansion coeflicient, the expansion coenicients of said parts differing not more than by slight values, the mass area values of themarts of the expansion member being different and so related to the coefilcients of expansion of said parts that upon a rapid change to a different temperature a temporary difierence of dimension occurs which, to a major extent, is eliminated upon that different temperature being maintained, and a magnet, the contacts of said switch being under the influence of said magnet.

5. In a thermostatic switch, an expansion member comprising two longitudinally expanding parts secured together at one end, a contact member adjacent the other end of said parts and adapted to be operated by said other end, each of said parts having an expansion coeflicient, the expansion coeflioients of said parts differing not more than by slight values, the mass area values of said parts being difierent and so related to the coeflicient of expansion of said parts that upon a rapid change to a diflferent temperature a temporary difference of dimension occurs which, to a major extent, is eliminated upon that different temperature being maintained, said difference of dimension causing actuation 0! said contact member, the slight differences in expansion coeillcients serving to maintain the switch in operated position when both elements have reached the same temperature.

6. A safety thermostat comprising a switch member and a heat-sensitive member comprising two longitudinally expanding parts each having an expansion coeflicient, the expansion coefiicients of said parts differing not more than by slight values, one of said parts comprising at least one thin outer member and the other part comprising a central inner member, said parts being anchored to one another at the ends remote from the switch member whereby upon a rapid change to a difierent temperature the thin outer part will first respond and move the central inner part from the switch member, the mass area values of said parts being so related to the coefilcients of expansion of said parts that on continued heating the central inner member will move back towards the switch member so that on reaching a steady temperature the relative dimension of the respective members is not more than slightly different from that'before heat was applied.

LEONARD SATCHWELL. 

